The present invention relates to a small variable-magnification type copying machine employing a zoom lens for magnification variation. More particularly, the invention relates to a device for adjusting the focal position of the zoom lens during assembly.
Recently, copying machines have been separately miniaturized and given a magnification varying function. However, no fully acceptable small variable-magnification type copying machine has heretofore been known. A copying machine employing a fiber lens array may satisfy the requirement for miniaturization; however, it is difficult to provide it with a magnification varying function. On the other hand, it is difficult to miniaturize and simplify the construction of a conventional copying machine with a magnification varying optical system using a fixed focus lens due to the requirement for a mechanism for correcting for the distance between the object and image during magnification variation.
FIG. 1 shows the arrangement of a small variable-magnification type copying machine using the aforementioned fixed focus lens. In this system, in order to miniaturize the system, an original holding stand 11 made of a glass plate is designed so as to be movable parallel to the plane of its surface on which an original O is placed. As the original holding stand 11 is moved, the original O on the stand 11 is illuminated by an illuminating lamp unit 12, as a result of which the slit image of the original O is formed on a photosensitive drum 17 by means of mirrors 13, 14a and 14b, a fixed focus lens 15, and a mirror 16.
In the copying machine, magnification is varied according to a method in which the fixed focus lens 15 is moved to a position corresponding to a desired magnification (as indicated by broken lines in FIG. 1), and the mirrors 14a and 14b are moved to positions (also indicated by broken lines) in association with the variation of the distance between the object and image due to the movement of the fixed focus lens 15.
The movement control mechanism needed for moving the fixed focus lens 15 and the mirrors 14a and 14b in the above-described manner is unavoidably intricate in construction, which limits the miniaturization of the copying machine.
On the other hand, a variable magnification type copying machine employing a zoom lens system can be made simple in construction because the distance between the object and image can be maintained constant due to the use of the zoom lens. An example of such a copying machine, as shown in FIG. 2, includes mirrors 18 and 19 provided below an original holding stand 11. The mirror 18 operates to reflect the slit image of an original O illuminated by an illuminating lamp unit 12 along a path parallel to the plane of the stand 11. The mirror 19 is used to reflect the slit image towards a photosensitive drum 17. The zoom lens 20 is provided between the mirrors 18 and 19.
In the above-described arrangement, the distance between the object and image can be maintained constant by changing the focal length of the zoom lens 20. Therefore, the mirrors 18 and 19 may be fixed in position. In addition, in the arrangement shown in FIG. 2, the original holding stand 11 is movable. Accordingly, the copying machine can be made small in size and it has a magnification varying function.
Even in this copying machine, however, the zoom lens 20 inevitably has manufacturing errors, and accordingly adjustment must be performed to correctly make the focal point coincide with the upper surface of the original holding stand 11, namely, the surface of the original, irrespective of the given copying magnification. This adjustment is carried out using the following two steps:
It is assumed that the copying machine has several magnifications. The position, of the whole zoom lens 20, and the position, in the direction of the optical axis, of one of the lens groups forming the zoom lens 20, are adjusted so that, when the focal length of the zoom lens is changed for the different magnifications, the focal positions are the same (or within the allowable depth of focus). If the focal position coincides with the upper surface of the original holding stand 11 (or the original's surface), then no further adjustment is needed. However, since the zoom lens 20 may have manufacturing errors as described above, in general, it is difficult to coincide the focal position with the upper surface of the original holding stand 11 by such a simple adjustment. It is now assumed that, as a result of the previous adjustment, the focal positions for the different magnifications are at the point A near the original holding stand 11. Under this condition, the mirror 18 is moved to adjust the optical path length so that the point A falls on the upper surface of the original holding stand 11. At this time, for all magnifications, the zoom lens 20 can be correctly focused on the original's surface.
However, the above-described adjustment performed in two steps cannot meet the requirements for simplifying the construction and assembly of the copying machine and miniaturizing the latter because it is intricate and requires a mechanism for adjusting the position of the mirror 18. Furthermore, the movement of the mirror 18 results in another difficulty that the original reading position is changed with movement of the mirror 18.